Emergency power supply

ABSTRACT

The disclosed embodiment of the present invention is an auxiliary power supply which generally includes a battery, a relay for connecting the battery to an output in the absence of line voltage, and a circuit for charging the battery. The charging circuit includes a circuit for providing a relatively low charge rate to the battery and another circuit for providing a relatively high charge rate to the battery. A Schmitt trigger circuit is responsive to the voltage level of the battery for activating and deactivating the low charge rate circuit. A gate compares the voltage level of the battery with a reference voltage level to control the high charge rate circuit. Once the high charge circuit has been rendered inoperative during a particular period between line voltage failures, it remains inoperative. An interrogator circuit periodically draws a relatively small energy current pulse from the battery, thereby placing it under a load condition while the Schmitt trigger is sensing the voltage level of the battery. If, by virtue of the load condition, the voltage level of the battery drops below one of the trigger levels of the Schmitt circuit, the trigger circuit will activate the low charge circuit to charge the battery.

United States Patent Beaman et a1.

[ Mar. 5, 1974 EMERGENCY POWER SUPPLY [75] Inventors: Don L. Beaman,Sunnyvale; John T.

Shoberg, Milpitas, both of Calif.

[73] Assignee: Shetec, Inc., San Jose, Calif.

[22] Filed: Feb. 9, 1973 [21] Appl. No.: 331,068

[52] US. Cl. 307/66, 320/39 [51] lint. Cl. .l H02j 7/00 [58] Field ofSearch 307/66, 43, 64; 320/39, 40;

[56] References Cited UNITED STATES PATENTS 3,659,181 4/1972 Bembenek320/39 Primary Examiner-Robert K. Schaefer Assistant ExaminerM. GinsburgAttorney, Agent, or FirmAndrew G. Pulles [57] ABSTRACT The disclosedembodiment of the present invention is an auxiliary power supply whichgenerally includes a battery, a relay for connecting the battery to anoutput in the absence of line voltage, and a circuit for charging thebattery. The charging circuit includes a circuit for providing arelatively low charge rate to the battery and another circuit forproviding a relatively high charge rate to the battery. A Schmitttrigger circuit is responsive to the voltage level of the battery foractivating and deactivating the low charge rate circuit. A gate comparesthe voltage level of the battery with a reference voltage level tocontrol the high charge rate circuit. Once the high charge circuit hasbeen rendered inoperative during a particular period between linevoltage failures, it remains inoperative. An interrogator circuitperiodically draws a relatively small energy current pulse from thebattery, thereby placing it under a load condition while the Schmitttrigger is sensing the voltage level of the battery. It, by virtue ofthe load condition, the voltage level of the battery drops below one ofthe trigger levels of the Schmitt circuit, the trigger circuit willactivate the low charge circuit to charge the battery.

, 15 Claims, 2 Drawing Figures TRANS. /2O RECTIFIER &F|LTER INDICATORSLOW HIGH /46 CHARGE CHARGE VOLTAGE 22 cIRcuIT cIRcuIT REGULATOR ,5

SCHMITT INTERRoGAToR TRIGGER 26' I l4 RELAY [16 I BATTERY l EMERGENCYPOWER SUPPLY This invention relates generally to an auxiliary powersupply and more particularly to an improved charging circuit for abattery in an auxiliary power supply.

Auxiliary power supplies are employed to supply emergency power forinstallations where the absence of line voltage would cause undesiredconditions, and in some instances cause a disaster. For example, anoperatin g room in a hospital must remain illuminated during anoperation. As a result, the majority of the betterequiped hospitalsemploy auxiliary power supplies as a source of power for the lightingsystem in the operating room during a period of line power failure.Railway signals and municipal fire alarm systems are other examples inwhich some form of electrical power must be made available during anelectrical power failure. Other examples include burgler alarm systemsand telephone exchanges, both of which must remain active during a powerfailure.

Telephone exchanges also require the use of stationary batteries, suchas found in auxiliary power supplies, for another purpose. The use ofstationary batteries in a telephone exchange prevents cross-talk betweendifferent telephone circuits. If a common power supply is employed, itis difficult to prevent one telephone conversation from beingtransferred to all other telephone circuits in the exchange through thecommon supply.

In each of these and in other situations where a stationary battery isrequired, there is a need for a system to automatically maintain thecharge of the battery at a particular level so that sufficient powerwill be available when the line voltage fails. The charge of a batterywill be reduced in the absence of any charge supplied thereto over aperiod of time. Stationary batteries have a self-discharge rate and,accordingly, must be periodically recharged or continuously tricklecharged to maintain a desired charge level. The self-discharge rate of aparticular battery is dependent upon a number of factors and usuallydiffers from one battery to another. The temperature of the batterygreatly affects its selfdischarge rate, with an increase in temperaturecausing an increase in the self-discharge rate. The shelf life of abattery also affects its self-discharge rate. Furthermore, batteries maybe susceptible to losing their charge through leakages. Accordingly, itcan be readily appreciated that the rate of charge loss of a particularbattery cannot be accurately determined.

It has been the principle practice in the past to supply, at arelatively low rate, a continuous charge, called a trickle charge, intothe battery to offset the charge loss therefrom. This technique requiresthat the charging rate supplied to the battery be equal to the rate ofcharge loss from the battery. Unfortunately, the rate of charge lossfrom the battery, as mentioned above, cannot be accurately determined,since it varies from one battery to another and in accordance withcertain variable factors. Since it is essential that the battery chargeremain at a relatively high level so that it will be available to supplythe necessary power upon demand, it has been the practice tooverestimate the rate of charge loss from a battery and, consequently,the charge to be supplied thereto. Furthermore, since the rateof chargeloss varies from one battery to another, it has been necessary in thepractice of this technique to select a charging rate which correspondsto the highest rate of charge loss which will be encountered in aparticular system and for any battery employed therein. This method ofdetermining the charging rate supplied to the battery, whethercontinuously or periodically, suffers from certain distinctdisadvantages.

It can be readily appreciated from the above discussion that only onrare occurrences will the selected charging rate be equal to the rate ofcharge loss from the battery. If the charging rate is selected so thatit is never less than the rate of charge loss, then a condition existsin which energy is supplied to the battery which does not add to itscharge. Any such energy supplied to a battery which does not add to itscharge will be con verted into heat and electrolysis. When energy isconsumed in the form of electrolysis, hydrogen and oxygen in gas formare produced and escape from the electrolyte solution. This condition,which is called outgassing, not only causes corrosion of the batterystructure, but also has a corrosive affect on structures in thesurrounding environment. Furthermore, the production and resultant lossof hydrogen and oxygen depletes the water from the electrolyte solution.As a result of this loss, water must be periodically supplied to thebattery, thereby increasing the maintenance and service costs associatedwith such systems.

The actual charge of a battery is measured in the amount of current thebattery can deliver over a predetermined time period. For example, if abattery can deliver five amperes for a period of twenty hours, it israted as a ampere-hour battery.

However, there is no means available for measuring the charge of abattery directly in units of amperehours or, more specifically, in theamount of current which the battery can deliver over a predeterminedperiod of time, without fully discharging the battery while measuringsuch quantities. Accordingly, such a measure of the charge level of abattery must be made by other means or techniques. One such techniqueinvolves measuring the specific gravity of the electrolyte in alead-acid storage battery. However, this technique is not susceptible ofimplementation with automatic detection means and without humanintervention.

Since stationary batteries are designed to maintain a relatively highand substantially constant voltage level after a relatively long currentdischarge, it is not possible to accurately measure the charge of abattery by sensing its voltage level under no load conditions. Aparticular battery may have reached a near discharged condition whileits voltage level remains relatively high without any load appliedthereto. Accordingly, a charging circuit which is responsive to thevoltage level of the battery under no load conditions cannot determinethe actual charge level of the battery. As a result, such a battery willnot be available to supply the required power when a line power failureoccurs.

Accordingly, it is a primary object of the present invention to providea charging circuit for the battery in an auxiliary power supply which iscapable of sensing the actual charge condition of the battery.

Another object of the present invention is to provide a charging circuitfor the battery in an auxiliary power supply which will maintain thebattery at a relatively high charge level so that it will be availableto provide the required amount of power at all times in the event of aline voltage failure.

A further object of the present invention is to provide a chargingcircuit for the battery in an auxiliary power supply which is controlledto supply no more energy thereto than that required to sustain thecharge of the battery at a particular level. These and other objects ofthe present invention are attained by the provision of means for sensingthe charge condition of the battery by placing it under load andsimultaneously sensing its voltage level.

A feature of the present invention resides in the provision of aninterrogator circuit for periodically interrogating the battery so thatits charge condition can be more acurately determined.

A further feature of the present invention resides in the provision ofmeans for periodically drawing a small energy current pulse from thebattery to place it under a load condition and means for sensing thevoltage level of the battery under such a load condition.

More specifically, the disclosed embodiment of the present inventionemploys an oscillator for periodically loading the battery, a Schmitttrigger connected to the battery and providing an output when itsvoltage drops to a predetermined level, and a switching circuit operablein response to the Schmitt trigger output to supply a charging currentto the battery. This arrangement insures that the battery will not becharged when its charge condition is high enough to cause its terminalvoltage to remain high under a loaded condition. As a result, thedisadvantages of the prior mentioned techniques are overcome.

The invention, however, as well as other objects, features andadvantages thereof will be more fully realized and understood from thefollowing detailed description, when taken in conjunction with theaccompanying drawings, wherein:

FIG. 1 is a block diagram of an auxiliary power supply constructed inaccordance with the principles of the present invention; and

FIG. 2 is a schematic diagram ofa portion of the system illustrated inFIG. 1.

With reference to FIG. 1, there is shown an auxiliary power supplyconstructed in accordance with the principles of the present inventionwhich is connected to a source of line voltage on a pair of terminalsand 12 and supplies an output on a pair of terminals 14 and 16. Wheneverthe line voltage on the terminals 10 and 12 fails, power is suppliedacross the terminals 14 and 16 which are disposed for connection to anoutput device, such as a lighting system.

The line voltage on the terminals 10 and 12 is connected through atransformer, rectifier and filter circuit 18 to supply unregulateddirect current voltage at an output thereof on a line 20. A voltageregulator 22 which is connected to the line supplies regulated directcurrent voltage at its output on a line 24. When the line voltage on theterminals 10 and 12 fails, no voltage will be supplied to the line 20. Arelay 26 closes a circuit between a battery 28 and the terminals 14 and16 in response to the absence of voltage on the line 20.

In order to assure that the battery 28 will be available to supplysufficient power to the terminals 14 and 16 when a power line failureoccurs, the battery must be maintained in a charged condition at alltimes. The battery 28 can be charged through either a low rate chargecircuit 30 or a high rate charge circuit 32 which are both responsive tocontrol signals to connect the voltage on the line 20 through associatedimpedances or current limiting elements to output lines 34 and 36,respectively. The lines 34 and 36 are connected through a diode 38 tothe positive terminal of the battery 28.

Thelow rate charge circuit 30 is connected to indicators 40 whichprovide an indication of the charging state of the circuit 30.

The regulated voltage on the line 24 is connected through a voltagedivider circuit, formed of resistors 42 and 44 connected in a series, toground potential to provide a reference voltage at a circuit point 46.The reference voltage at the circuit point 46 is connected to the highrate charge circuit 32 which compares that voltage with the voltage onthe line 36 to control the charge supplied therethrough to the battery28.

A Schmitt trigger circuit 48 is responsive to the voltage level of thebattery 28 and supplies an appropriate control signal on an output line50 to control the low charge circuit 30. An interrogator circuit 52periodically interogates the battery 28 by drawing a relatively highcurrent pulse of short duration therefrom to effectively place thebattery 28 under a load condition. During the time that the battery 28is placed under the load condition by the interrogator 52, the Schmitttrigger 48 is operative to sense its voltage level. If the loadcondition on the battery 28 causes its voltage level to drop below thetrigger level of the Schmitt trigger circuit 48, it will provide anappropriate control signal on the line 50 to energize the low ratecharge circuit 30.

Since the Schmitt trigger circuit 48 and the interrogator circuit 52 areprovided with power from the line 24, the battery 28 will, with theexception of its connection through the relay 26, not be under any loadcondition during a line voltage failure. More particularly, during aline voltage failure the interrogator 52 will not be operative to draw acurrent pulse from the battery 28 due to the lack of voltage on the line24. The particulars of auxiliary power supply illustrated in FIG. 1will, however, be more fully understood from the description of FIG. 2hereinbelow.

As shown in FIG. 2, the battery 28 is connected in series with a fuse 56between a circuit point 58 and ground potential. The circuit point 58 isconnected through the relay 26 to the terminal 14. The relay 26 includesa transistor 60 having its emitter connected to the circuit point 58 andits collector connected to the terminal 14 and through a capacitor 62 toground potential. The direct current voltage on the line 20 is connectedthrough a diode 64 and a pair of resistors 66 and 68 to groundpotential. The base of the transistor 60 is connected to the junctionbetween the resistors 66 and 68 which form a voltage divider circuit.When the line voltage fails, the voltage on the line 20 drops to groundpotential, thereby forward biasing the transistor 60 and permittingcurrent flow therethrough from the battery 28 to the terminal 14.

Through various factors, the charge of the battery diminishes over aperiod of time unless it is recharged. The charging circuits 30 and 32provide such a recharge for the battery 28 to maintain its charge levelabove a predetermined value at all times. The low charge circuit 30includes a transistor 70 having its collector connected through aresistor 72 to the voltage on line 20. The emitter of the transistor 70is connected through the diode 38 to the positive side of the battery28. The high charge circuit 32 includes a transistor 74 having itscollector connected through a resistor 76 to the voltage on line 20. Theemitter of the transistor 74 is connected through the diode 38 to thepositive side of the battery 28. The value of the resistor 72 isselected such that when the transistor 70 is conductive,

a relatively small current is supplied to the battery 28. The value ofthe resistor 76 is selected such that when the transistor 74 isconductive a relatively high current is supplied to the battery 28.

The conduction of the transistor 74 is controlled by a circuit whichincludes a transistor 78 having its emitter maintained at the voltagelevel of the battery 28 by virtue of its connection to the anode of thediode 38. The base of the transistor 78 is connected to the circuitpoint 46. Accordingly, when the voltage on the emitter of the transistor78 is less than the voltage at the circuit point 46, the transistor 78is non-conductive.

The collector of the transistor 78 is connected to the gate of a siliconcontrolled rectifier (SCR) 80 which is connected in series with aresistor 82 between the voltage on the line and ground potential. Thebase of the transistor 74 is connected to the anode of the SCR 80.Accordingly, when the SCR 80 is non-conductive, the transistor 74 willbe rendered conductive, thereby supplying a charging current to thebattery 28. When the voltage level of the battery 28 attains apredetermined value, the transistor 78 will be rendered conductivethereby placing a positive voltage on the gate electrode of the SCR 80.As a result of the conduction of the transistor 78, the SCR 80 becomesconductive and the bias is removed from the transistor 74 rendering itnonconductive.

Once the SCR 80 becomes conductive, it remains in that state until thevoltage on the line 20 reduces to near ground potential. Accordingly,the battery 28 will receive a high charge from the circuit 32 only onceduring any period between power line failures or if the line voltage isremoved for'any reason. In an example of this invention, the voltage atcircuit point 46 was established at 14.2 volts with an approximate rangeof from 12.6 volts to 14.4 volts at the circuit point 58.

The indicator circuit 40 includes a pair of transistors 84 and 86connected in series with indicator lamps 88 and 90, respectively. Thebase of the transistor 86 is connected through a resistor 92 to thecollector of the transistor 70, such that the transistor 86 is renderedconductive to illuminate the lamp 90 whenever the transistor 70 isconductive. The base of the transistor 84 is connected to the emitter ofthe transistor 86 through a resistor 94 such that the transistor 84 isrendered conductive whenever the transistor 86 is nonconductive.Conduction of the transistor 86 causes the lamp 90 to illuminateindicating that the low charge circuit is providing a charging currentto the battery 28. When the transistor 70 is rendered non-conductive toremove the charging current from the battery 28, the transistor 86 isalso rendered non-conductive and the transistor 84 is renderedconductive to cause the lamp 88 to illuminate to indicate that thebattery 28 is not receiving a charging current from the low chargecircuit 30.

The conduction of the transistor 70 in the low charge circuit 30 iscontrolled by an output of the Schmitt trigger circuit 48. The Schmitttrigger circuit 48 includes a transistor 96 having its base connected tothe positive side of the battery 28 through a resistor 98. The collectorof the transistor 96 is connected through a resistor 100 to the base ofthe transistor 70 and through a resistor 102 and a capacitor 104 inparallel with one another to the base of a transistor 106. The base ofthe transistor 106 is also connected through a resistor 108 to groundpotential. The emitters of the transistors 96 and 106 are connected toone another and through a resistor 110 to ground potential.

When the voltage level of the battery 28 is at a relatively low value,the transistor 96 is non-conductive, thereby applying a control signalto the transistor 70 to render it conductive. Because of the voltagelevel of the emitters of the transistors 96 and 106 when the transistor96 is non-conductive, the voltage at the circuit point 58 must raise toa predetermined value before the transistor 96 will be renderedconductive. Conduction of the transistor 96 causes the transistor 106 tobe rendered non-conductive, thereby lowering the voltage level at theemitter thereof. Accordingly, the transistor 96 is rendered conductiveat one level and nonconductive at a lower level of voltage at thecircuit point 58. In a typical example, the transistor 96 is renderedconductive when the circuit point 58 attains a level of 14.4 volts andis rendered non-conductive when the voltage at the circuit point 58reduces to 12.6 volts. Accordingly, in such an example, when the voltage of the battery 28 is equal to or less than 12.6 volts, the chargingcircuit 30 will supply a charging current thereto until the voltage ofthe battery 28 raises above 14.4 volts.

In the preferred embodiment of this invention, the interrogator circuit52 includes an oscillator having a unijunction transistor 112. The baseof the transistor 112 is connected through resistor 114 to the regulatedvoltage on line 24 and through a capacitor 116 to ground potential. Thecapacitor 116 charges through the resistor 114 until it attains aparticular voltage level corresponding to the trigger level of thetransistor 112. The transistor 112 is connected through a resistor 118to the regulated voltage on the line 24 and through a pair of resistors120 and 122 to ground potential. When the transistor 112 triggers, thecharge is removed from the capacitor 116 and a positive voltage isdeveloped across the resistors 120 and 122. The junction of theresistors 120 and 122 is connected to the base of a transistor 124 whichis rendered conductive whenever the transistor 112 triggers. In atypical example, the parameters of the oscillator can be selected toprovide a voltage pulse at the base of the transistor 124 once every 15seconds.

The emitter of the transistor 124 is connected to ground potential andthe collector thereof is connected through a pair of resistors 126 and128 to the positve side of the battery 28. A capacitor is connected inparallel with the resistor 126 and forms a parallel time constantcircuit therewith and a series time constant circuit with the resistor128.

When the transistor 124 is rendered conductive, a current path isestablished from the positive side of the battery 28 through theresistor 128, the capacitor 130 and the transistor 124 to groundpotential. The time constant of the resistor 128 and the capacitor 130are such that a relatively short duration current pulse is drawn fromthe battery 28. In a typical example, the RC time constant of theresistor 128 and capacitor 130 is approximately 3 milliseconds and theRC time constant of the resistor 126 and the capacitor 130 isapproximately 5 seconds. When the transistor 124 is renderednon-conductive, the capacitor 130 discharges through the resistor 126.

The short duration current pulse which is drawn from the battery 28effectively loads the battery 28. If the charge of the battery 28 isreduced from its maximum,

the voltage level thereof may drop sufficiently to trigger the Schmitttrigger circuit 48, thereby energizing the low charge circuit 30. In theabove given example, if the current pulse drawn from the battery 28reduces its voltage to less than 12.6 volts, the Schmitt trigger circuit48 will provide the proper bias to the transistor 70 to render itconductive which will, in turn, supply a charging current to the battery28. As previously mentioned, once the Schmitt trigger circuit 48 istriggered to cause transistor 70 to conduct, it will remain in thatstate until the input thereto raises to 14.4 volts.

In a typical example of this invention, the maximum amplitude of thecurrent pulse was approximately 4 amperes. This amount of current havinga decay time established by the RC time constant of the circuit loadsthe battery sufficiently for its terminal voltage to drop if its chargelevel is reduced, but does not remove any significant amount of energytherefrom. ln the above example, the charge of the battery would reduceonly about 5 percent over a 6 month period in the absence of any chargebeing supplied thereto.

If the resistor 126 and capacitor 130 are removed from the circuit andthe resistor 128 is connected directly to the collector of thetransistor 124, the circuit would operate properly. in such aconfiguration, a rectangular pulse of current would be produced. Thecapacitor 130, therefore, permits a smaller amount of energyto bewithdrawn from the battery 28 with the same maximum amplitude of currentor loading effect. Furthermore, the capacitor 130 provides protection tothe battery in the event of a failure of transistor 124. Accordingly, itcan be appreciated that this and other changes can be made in theinvention without departing from the scope of the appended claims.

The invention claimed is:

v 1. In an auxiliary power supply for connecting a battery to an outputin the absence of line voltage, a circuit for charging the batterycomprising a. a source of charging voltage,

b. means for connecting said source to the battery,

c. means for sensing the voltage level of the battery and energizingsaid connecting means in response thereto, and

d. means for loading the battery for a relatively short time durationduring the presence of said line voltage, whereby said sensing means isoperative to sense the voltage level of the battery under a loadedcondition.

2. A circuit for charging the battery in an auxiliary power supply asdefined in claim ll, further comprising means for periodicallyenergizing said loading means.

3. A circuit for charging the battery in an auxiliary power supply asdefined in claim ll, wherein said loading means includes means fordrawing a pulse of current from the battery.

4. A circuit for charging the battery in an auxiliary power supply asdefined in claim 3, wherein said loading means further includes aswitching element connecting said current drawing means across saidbattery.

5. A circuit for charging the battery in an auxiliary power supply asdefined in claim 4, further comprising means for periodically energizingsaid switching element.

5. A circuit for charging the battery in an auxiliary power supply asdefined in claim 5, wherein said current drawing means includes aresistor and capacitor connected in series with one another and withsaid switching element.

7. A circuit for charging the battery in an auxiliary power supply asdefined in claim 5, wherein said energizing means includes anoscillator.

8. A circuit for charging the battery in an auxiliary power supply asdefined in claim 1, wherein said sensing means includes a triggercircuit responsive to one voltage level for switching from a firstoutput state to a second output state and responsive to another voltagelevel for switching from the second to the first output state, meansresponsive to one output state of said trigger circuit for connectingsaid charging source across the battery and responsive to the otheroutput state of said trigger circuit for disconnecting said chargingsource from the battery.

9. A circuit for charging the battery in an auxiliary power supply asdefined in claim 8, wherein said loading means includes means fordrawing a current pulse from the battery and means for periodicallyenergizing said current drawing means.

10. A circuit for charging the battery in an auxiliary power supply asdefined in claim 9, wherein said current drawing means includes a firstresistor and :1 capacitor connected in series with one another, a secondresistor connected in parallel with said capacitor, and a switchingelement connecting the series connected first resistor and capacitoracross the battery, and wherein said energizing means includes means forproducing a pulse output having a predetermined period for periodicallyenergizing said switching element, the time constant of said secondresistor and said capacitor being less than the period of said pulseoutput and being greater than the time constant of said first resistorand said capacitor.

111. A circuit for charging the battery in an auxiliary power supply asdefined in claim ll, wherein said connecting means includes a highcharge rate circuit for connecting said source of charging voltage tothe battery.

12. A circuit for charging the battery in an auxiliary power supply asdefined in claim 11, further comprising means connected to said highcharge rate circuit for comparing the voltage level of the battery witha reference voltage and for controlling said high rate circuit inaccordance therewith.

13. A circuit for charging the battery in an auxiliary power supply asdefined in claim 12, further comprising means for disabling said highcharge rate circuit after the voltage of the battery attains apredetermined value.

14. A circuit for charging the battery in an auxiliary power supply asdefined in claim 1, wherein said connecting means includes a firstimpedance element connectable between saidsource and the battery, and ahigh charge rate circuit including a second impedance elementconnectable between said source and the battery and having a value ofimpedance less than the impedance value of said first impedance element,said high charge rate circuit including a switching element connectingsaid second impedance element between said source and the battery in onestate thereof, said means for sensing the voltage level of the batteryproviding a control signal when voltage of the battery is less than apredetermined value, said switching element being responsive to saidcontrol signal for changing to said one state thereof.

15. A circuit for charging the battery in an auxiliary means fordisabling said high charge circuit after the power supply as defined inclaim 14, further comprising voltage of the battery attains a certainvalue higher memory means connecting said control signal to said thansaid predetermined value. switching element, said memory means including

1. In an auxiliary power supply for connecting a battery to an output inthe absence of line voltage, a circuit for charging the batterycomprising a. a source of charging voltage, b. means for connecting saidsource to the battery, c. means for sensing the voltage level of thebattery and energizing said connecting means in response thereto, and d.means for loading the battery for a relatively short time durationduring the presence of said line voltage, whereby said sensing means isoperative to sense the voltage level of the battery under a loadedcondition.
 2. A circuit for charging the battery in an auxiliary powersupply as defined in claim 1, further comprising means for periodicallyenergizing said loading means.
 3. A circuit for charging the battery inan auxiliary power supply as defined in claim 1, wherein said loadingmeans includes means for drawing a pulse of current from the battery. 4.A circuit for charging the battery in an auxiliary power supply asdefined in claim 3, wherein said loading means further includes aswitching element connecting said current drawing means across saidbattery.
 5. A circuit for charging the battery in an auxiliary powersupply as defined in claim 4, further comprising means for periodicallyenergizing said switching element.
 6. A circuit for charging the batteryin an auxiliary power supply as defined in claim 5, wherein said currentdrawing means includes a resistor and capacitor connected in series withone another and with said switching element.
 7. A circuit for chargingthe battery in an auxiliary power supply as defined in claim 5, whereinsaid energizing means includes an oscillator.
 8. A circuit for chargingthe battery in an auxiliary power supply as defined in claim 1, whereinsaid sensing means includes a trigger circuit responsive to one voltagelevel for switching from a first output state to a second output stateand responsive to another voltage level for switching from the second tothe first output state, means responsive to one output state of saidtrigger circuit for connecting said charging source across the batteryand responsive to the other output state of said trigger circuit fordisconnecting said charging source from the battery.
 9. A circuit forcharging the battery in an auxiliary power supply as defined in claim 8,wherein said loading means includes means for drawing a current pulsefrom the battery and means for periodically energizing said currentdrawing means.
 10. A circuit for charging the battery in an auxiliarypower supply as defined in claim 9, wherein said current drawing meansincludes a first resistor and a capacitor connected in series with oneanother, a second resistor connected in parallel with said capacitor,and a switching element connecting the series connected first resistorand capacitor across the battery, and wherein said energizing meansincludes means for producing a pulse output having a predeterminedperiod for periodically energizing said switching element, the timeconstant of said second resistor and said caPacitor being less than theperiod of said pulse output and being greater than the time constant ofsaid first resistor and said capacitor.
 11. A circuit for charging thebattery in an auxiliary power supply as defined in claim 1, wherein saidconnecting means includes a high charge rate circuit for connecting saidsource of charging voltage to the battery.
 12. A circuit for chargingthe battery in an auxiliary power supply as defined in claim 11, furthercomprising means connected to said high charge rate circuit forcomparing the voltage level of the battery with a reference voltage andfor controlling said high rate circuit in accordance therewith.
 13. Acircuit for charging the battery in an auxiliary power supply as definedin claim 12, further comprising means for disabling said high chargerate circuit after the voltage of the battery attains a predeterminedvalue.
 14. A circuit for charging the battery in an auxiliary powersupply as defined in claim 1, wherein said connecting means includes afirst impedance element connectable between said source and the battery,and a high charge rate circuit including a second impedance elementconnectable between said source and the battery and having a value ofimpedance less than the impedance value of said first impedance element,said high charge rate circuit including a switching element connectingsaid second impedance element between said source and the battery in onestate thereof, said means for sensing the voltage level of the batteryproviding a control signal when voltage of the battery is less than apredetermined value, said switching element being responsive to saidcontrol signal for changing to said one state thereof.
 15. A circuit forcharging the battery in an auxiliary power supply as defined in claim14, further comprising memory means connecting said control signal tosaid switching element, said memory means including means for disablingsaid high charge circuit after the voltage of the battery attains acertain value higher than said predetermined value.